This invention relates to a DC-to-DC voltage increasing power source of a pumping capacitor system and, more particularly, to such power source capable of preventing occurrence of unnecessary radiation due to abrupt flowing of a large current from a DC power source.
It sometimes becomes necessary in an audio device mounted on an automobile or a portable type audio device to obtain a DC voltage which is higher than a DC voltage produced by a battery or dry cell.
For converting a DC voltage from the power source to a higher DC voltage, there have been known a device using a BTL circuit, one using a DC-to-DC converter or a switching regulator and one using pumping capacitors.
The BTL circuit can produce, theoretically, an output voltage which is twice as large as the power source voltage (output power is four times as large as the power from the power source). Since, however, a load current flows in two amplifiers in this circuit, power loss in the amplifiers becomes double with resulting drop in efficiency. Besides, an output voltage larger than this double voltage cannot be obtained by this circuit.
A DC-to-DC converter and a switching regulator once switch a DC power source voltage with a high frequency and converts the voltage by a transformer and then restores the converted voltage to a DC voltage. In these devices, it is easy to obtain an output voltage which is larger than a voltage twice as large as the power source voltage, but their efficiency is poor because their transformer has copper loss and iron loss. Besides, since the transformer is relatively large and heavy, the device employing the transformer becomes bulky and heavy. There has been an attempt to overcome this disadvantage by increasing the switching frequency and thereby reducing the size of the transformer but, although the transformer becomes smaller, it becomes expensive because it requires a core with a small loss. Moreover, if the switching frequency increases, a switching element having a higher switching speed corresponding to the higher frequency must be employed and this results in increase in the cost of the switching element and necessity for a more complicated circuit design.
According to the device using the pumping capacitor system, a rectifying element such as a diode or a transistor is combined with a capacitor and a DC voltage from a power source is charged in the capacitor through the rectifying element and the capacitor is serially connected to the DC power source by switching or the like means to obtain a DC voltage which is higher than the DC power source voltage. According to the pumping capacitor system, a transformer is unnecessary so that the device can be made small and light and its manufacturing cost is reduced. Moreover, a capacitor is higher in efficiency than a transformer because the capacitor is much smaller in equivalent serial resistance than the transformer which has copper loss and iron loss. The pumping capacitor system therefore is advantageous in that a DC-to-DC voltage-increasing device can be constructed easily and cheaply.
FIG. 2 shows a prior art pumping capacitor type voltage-increasing power source. This circuit comprises charge and discharge means 12 and switching means 10. The switching means 10 comprises complementary push-pull transistors 14, 15 and complementary push-pull transistors 16, 17 connected respectively to a DC power source 11 (voltage value being E). The complementary transistors 14, 15 and 16, 17 are respectively driven by drive signals A and Awhich are in 180 degree phase relationship whereby the potentials at output lines 18 and 20 and switched alternately between E and ground.
The charge and discharge means 12 is made of combination of capacitors 21 through 26 and diodes 27 through 32. While voltage 0 is applied to an input terminal 33 and voltage E to an input terminal 34, the capacitor 21 is charged to voltage E through the diode 27 whereas the capacitor 23 is charged to voltage E through the diode 30. While voltage E is applied to the input terminal 33 and voltage 0 to the input terminal 34, sum voltage 2E of the power source voltage E and the voltage E of the capacitor 21 is applied to the capacitor 22 through the diode 28 so that this capacitor 21 is charged to 2E. Sum voltage 2E of the power source voltage E and the voltage E of the capacitor 23 is applied to the capacitor 24 through the diode 31 to charge this capacitor 24 to 2E.
While voltage 0 is applied to the input terminal 33 and voltage E to the input terminal 34, sum voltage 3E of the power source voltage E and the voltage 2E of the capacitor 22 is applied to the capacitor 25 through the diode 29 to charge this capacitor 25 to 3E and this voltage 3E is provided at an output terminal 35.
During the same period, voltage 2E of the capacitor 24 is applied to the capacitor 26 through the diode 32 to charge this capacitor 26 to 2E and its terminal voltage-2E is provided at an output terminal 36.
The circuit of FIG. 2 is so designed that the on-off operation is performed with the transistors 14, 15 and 16, 17 being in a completely saturated state and, accordingly, inrush current for charging the capacitors 21 through 26 is generated in a period immediately after turning-on of the power source and during sharp increase in the load current with a result that a pluse-like electromagnetic radiation is produced which causes a high frequency noise, i.e., noise due to unnecessary radiation.
Further, in the circuit of FIG. 2, there tends to be produced a period of time, though it is only of a short duration, during which both the transistors 14 and 16, or both the transistors 16 and 17 are turned on due to accumulated charge in a transient period between the on state and off state of the transistors 14, 15 or the transistors 16, 17 with a resulting flow of a longitudinal current. This longitudinal current is a kind of inrush current and causes damage to component elements and pulse-like electromagnetic radiation resulting in generation of the high frequency noise.
It is, therefore, an object of the invention to provide a DC-to-DC voltage-increasing power source capable of preventing occurrence of the inrush current in a period immediately after turning on of a power source and during sharp increase in load current, or the inrush current due to the longitudinal current and thereby reducing noise caused by unnecessary radiation.